Babcock-Leighton solar dynamo: the role of downward pumping and the equatorward propagation of activity

TL;DR

This paper explores how downward magnetic pumping influences the Babcock-Leighton solar dynamo, enabling equatorward activity propagation and sustaining an 11-year cycle without deep meridional flow.

Contribution

It demonstrates that magnetic pumping near the surface can produce solar cycle features traditionally attributed to deep meridional flow, offering an alternative dynamo mechanism.

Findings

Downward pumping causes poloidal fields to become radial near the surface.

Equatorward migration of toroidal fields occurs via dynamo wave without deep flow.

Magnetic pumping allows for larger diffusivities while maintaining the cycle.

Abstract

The key elements of the Babcock-Leighton dynamos are the generation of poloidal field through decay and dispersal of tilted bipolar active regions and the generation of toroidal field through the observed differential rotation. These models are traditionally known as flux transport dynamo models as the equatorward propagations of the butterfly wings in these models are produced due to an equatorward flow at the bottom of the convection zone. Here we investigate the role of downward magnetic pumping near the surface using a kinematic Babcock-Leighton model. We find that the pumping causes the poloidal field to become predominately radial in the near-surface shear layer, which allows the negative radial shear to effectively act on the radial field to produce a toroidal field. We observe a clear equatorward migration of the toroidal field at low latitudes as a consequence of the dynamo wave even when there is no meridional flow in the deep convection zone. Both the dynamo wave and the flux transport type solutions are thus able to reproduce some of the observed features of the solar cycle, including the 11-year periodicity. The main difference between the two types of solutions is the strength of the Babcock-Leighton source required to produce the dynamo action. A second consequence of the magnetic pumping is that it suppresses the diffusion of fields through the surface, which helps to allow an 11-year cycle at (moderately) larger values of magnetic diffusivity than have previously been used.